The invention relates to a method for operating a boosted internal combustion engine, in particular a self-igniting internal combustion engine having direct fuel injection.
In self-igniting internal combustion engines homogeneous, lean fuel-air mixtures are often made to self-ignite, so that a high efficiency and improved exhaust emissions are obtained. A specific quantity of exhaust gas is retained in the combustion chamber in order to influence the mixture temperature at the end of a compression phase of the internal combustion engine. In the lower load and speed range the temperature level in the combustion chamber falls, making it difficult to regulate the mixture temperature on the basis of the smaller fuel mass involved in the reaction. One way of compensating for the energy loss due to falling exhaust gas temperatures is to increase the exhaust gas retention rate.
Despite a high rate of exhaust gas retention, however, below a certain exhaust gas temperature level it is no longer possible to ensure stable combustion. The reason for this is that the retained exhaust gas is basically slow to react, resulting in a combustion lag. This leads to increased exhaust emissions, which reduce the combustion efficiency and also lead to large mean pressure fluctuations of the internal combustion engine.
DE 198 10 935 C2 discloses a method for operating a four-stroke internal combustion engine, in which a homogeneous, lean basic mixture of air, fuel and retained exhaust gas is formed, which is burned by a compression ignition. At the same time an activation phase is interposed in order to extend the engine operating range with compression ignition. During the compression of the retained exhaust gas an activation fuel quantity is injected into the combustion chamber and is distributed as homogeneously as possible with the rest of the fuel fractions in the combustion chamber. Power output and compression impart thermal energy to the fuel, so that a chemical reaction or ignition is initiated at the gas exchange dead center.
The aforementioned method presupposes that a combustion, in which sufficient exhaust gases are produced at a high temperature, occurs in each working cycle. Since the self-ignition of a homogeneous, lean mixture depends very heavily on the engine parameters and the ambient conditions, misfiring can occur, which in extreme cases leads to a complete absence of combustion.
The object of the invention therefore is to create a method for operating an internal combustion engine in which a reliable, self-igniting operation is ensured.
According to the invention this object is achieved by a method that includes the steps of delivering a main combustion air quantity and a main fuel quantity, from which a main mixture is formed, to the combustion chamber; igniting the main mixture formed in an area of a ignition top dead center; and introducing an additional combustion air quantity and an additional fuel quantity into the combustion chamber after the combustion of the main mixture in such a way that a fuel-exhaust gas/air mixture is formed, which mixture is reacted in an area of a gas exchange top dead center of the piston.
A distinctive feature of the method according to the invention is that after combustion of the main mixture an additional combustion air quantity and an additional fuel quantity are introduced into the combustion chamber in such a way that a fuel-exhaust gas/air mixture is formed, which is reacted in an area of a gas exchange top dead center of the piston. An interim mixture is thereby formed for raising the combustion chamber temperature, said mixture being reacted by a compression ignition and/or applied ignition prior to the main combustion taking place, in such a way as to permit regulation of the main mixture temperature.
In a development of the invention the additional fuel quantity is introduced into the combustion chamber in an area between the end of a piston expansion stroke and a final part of a piston exhaust stroke. This ensures that the additional fuel quantity is distributed and vaporized in the combustion chamber well before the gas exchange dead center.
According to a further development of the invention the additional fresh air quantity is delivered to the combustion chamber in an area between a final part of the piston expansion stroke and a final part of the piston exhaust stroke. By introducing the additional fresh air quantity into the combustion chamber, an ignitable mixture is formed, the additional fresh air quantity being delivered in proportion to the additional fuel quantity. The exhaust gas energy raises the temperature of the interim mixture to a specific temperature level, the temperature of the mixture being determined by the proportions of fresh air and exhaust gas.
In a further development of the invention at least one exhaust valve and at least one inlet valve are opened during the introduction of the additional fresh air quantity and/or the additional fuel quantity. The exhaust valve is preferably opened first and then the inlet valve. This opening sequence causes a proportion of the exhaust gas to be expelled from the combustion chamber first, so that the introduction of an additional fresh air quantity is ensured by the pressure that has built up in the intake pipe.
Further features and combinations of features are set forth in the description. Actual exemplary embodiments of the invention are represented in simplified form in the drawings and are explained in more detail in the following description.